Version 1.3.0

Added

new vignettes:
- Package Software Architecture
- Process Graph Building Concepts
- Process Graph Building Application
- Developer Implementation Details
added the default openEO processes v1.2.0 as test data to the package in the RDS file format (system.file("openeo_processes/openeo_process_1.2.0.rds")) and added a test case for process parsing
active_process_list() and active_process_collection() which refer to the currently connected back-ends process list as well as the package interpretation of those process descriptions. See environment variables process_list and process_collection
active_data_collection() to fetch the data collection of the currently active back-end, which refers to the package variable data_collection
vignette about the software and package architecture and design choices based on API requirements

added line breaks if there are multiple validation messages of ProcessNodes
describe_process() also works now with Process and ProcessNode objects
use the active process list when looking up the description of an openEO process
package now depends on R > 3.5.0, because of RDS object de-/serialization (openEO test processes)
added a warning and a note for compute_result() if neither parameter 'format' of the function was set nor process 'save_result' was used in the process graph
refresh data collections after separate login as part of #83
improved disconnection behavior by adding a disconnect() function and a logout and disconnect on package unload
decoupled processes and data collection from the OpenEO connection
moved the setup of the RStudio connection observer into a separate helper function
naming of the vignettes to achieve an order in the vignette index entries

variables() function now returns the extracted variables correctly
coercion problems from checking with is.na on values with length > 1 have been fixed by using rlang::is_na #130, #123
an issue when calling the JSON serialization of class Process

removed utility functions .getProcessCollection() and .find_var_in_stack() which are replaced by the active_process_collection() function
functions to call data collection ($getCollectionNames(), $getDataCollection()) from connection as well as the fields (private$data_collection)
functions ($getProcessCollection()) and fields (private$process_collection, $processes) to store the process list and process collection